Direct methanol fuel cell liquid fuel with water and methanol and direct methanol fuel cell cartridge

ABSTRACT

There is provided a direct methanol fuel cell liquid fuel including methanol and water to be supplied to an anode of a direct methanol fuel cell, in which an electrical resistance of the liquid fuel is 5×10 5  Ω·cm or more and 1×10 7  Ω·cm or less at 25° C. There is provided a direct methanol fuel cell cartridge storing the direct methanol fuel cell liquid fuel, and a direct methanol fuel cell system. Such structures can provide a direct methanol fuel cell liquid fuel, a direct methanol fuel cell cartridge, and a direct methanol fuel cell system all appropriately used for stable electricity generation over a long period of time.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a division of application Ser. No. 11/252,602, filedOct. 19, 2005, which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a direct methanol fuel cell liquidfuel, to a direct methanol fuel cell cartridge using the liquid fuel ofthe present invention, and to a direct methanol fuel cell system usingthe liquid fuel of the present invention. The present invention morespecifically relates to a direct methanol fuel cell liquid fuel, whichhas a reduced total content of components adversely affectingelectricity generation efficiency for improving electricity generationperformance of a fuel cell.

2. Related Background Art

A polymer electrolyte fuel cell generally has a layered structure inwhich a proton conductive polymer electrolyte membrane is held betweenan anode (fuel electrode) and a cathode (oxidizer electrode). The anodeand the cathode are each composed of a mixture of: a catalyst having anoble metal such as platinum or an organometallic complex carried onconductive carbon; an electrolyte; and a binder. A fuel supplied to theanode passes through fine pores of the anode, reaches the catalyst, andreleases electrons by the action of the catalyst to convert intohydrogen ions. The hydrogen ions pass through the electrolyte membraneprovided between the electrodes, reach the cathode, and react withoxygen supplied to the cathode and electrons flowing from an externalcircuit into the cathode, to thereby produce water. The electronsreleased from the fuel pass through the catalyst and the conductivecarbon carrying the catalyst in the anode, are guided to the externalcircuit, and flow into the cathode from the external circuit. As aresult, in the external circuit, the electrons flow from the anode tothe cathode such that electric power is taken out.

Of the fuel cells, a direct methanol fuel cell (DMFC) allowingelectricity generation by mere direct supply of a liquid fuel containingmethanol and water has particularly attracted attention because of itsadvantages such as easy and safe handling of the fuel, and simple fuelstorage. Further, applications of the DMFC are expected in variousfields such as portable electrical devices.

Japanese Patent Application Laid-Open No. 2001-93551 discloses that afuel storage vessel for DMFC is formed of a removable vessel or a vesselthat may be refilled with a liquid fuel, to thereby allow size reductionand long-term drive of a DMFC system.

However, in actual long-term drive of this kind of DMFC system, traceamounts of impurity components in an aqueous methanol solution to besupplied as a fuel accumulate in a fuel cell. The accumulation of theimpurity components provides adverse effects such as deterioration ofcatalyst performance, thereby causing a problem in that an electromotiveforce of the fuel cell becomes unstable. The problem can be presumablysolved by removing the impurity components in the liquid fuel inadvance. However, which impurity components in the liquid fuel must bereduced to what extent are hitherto not known for realizing long-termstable drive of DMFC.

SUMMARY OF THE INVENTION

The inventors of the present invention have focused on components in aliquid fuel except methanol and water, and have repeated variousanalytical experiments. As a result, the inventors of the presentinvention have found that main impurity components in the liquid fuelinclude ketone components, aldehyde components, halogen components, andalkali metal components. Further, the inventors of the present inventionhave also found that the liquid fuel having the impurity componentsremoved therefrom exhibits an electrical resistance within a specificrange under normal temperatures.

In view of solving the above-mentioned problems, an object of thepresent invention is to provide a liquid fuel allowing stable long-termelectricity generation by DMFC, a DMFC cartridge, and a DMFC system.

According to a first aspect of the present invention, a direct methanolfuel cell liquid fuel includes methanol and water to be supplied to ananode of a direct methanol fuel cell, in which an electrical resistanceof the liquid fuel is 5×10⁵ Ω·cm or more and 1×10⁷ Ω·cm or less at 25°C.

A total content of components in the liquid fuel except methanol andwater is preferably 10 ppm or less. A total content of ketone componentsand aldehyde components in the liquid fuel is preferably 5 ppm or less.

A total content of halogen components in the liquid fuel is preferably 1ppm or less. A total content of alkali metal components in the liquidfuel is preferably 5 ppm or less.

According to a second aspect of the present invention, a direct methanolfuel cell cartridge stores the above-mentioned direct methanol fuel cellliquid fuel. According to a third aspect of the present invention, adirect methanol fuel cell system includes: (A) one of: a fuel cell unitincluding a proton conductive polymer electrolyte membrane held betweenan anode and a cathode, a passage for supplying a liquid fuel to theanode, and a passage for supplying an oxidizer gas to the cathode; and afuel cell stack obtained by stacking a plurality of fuel cell units; and(B) a fuel cartridge storing the liquid fuel to be supplied to theanode, in which the liquid fuel is the above-mentioned direct methanolfuel cell liquid fuel.

A material for a member to be brought into contact with the liquid fuelpreferably contains no components soluble in methanol or water.

The present invention can provide a direct methanol fuel cell liquidfuel, a direct methanol fuel cell cartridge, and a direct methanol fuelcell system all appropriately used for stable electricity generationover a long period of time.

Other features and advantages of the present invention will be apparentfrom the following description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGURE is a schematic diagram showing a structure of a direct methanolfuel cell system according to Example 4 of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described inmore detail.

(Characteristics of Liquid Fuel According to the Present Invention)

According to an embodiment of the present invention, a direct methanolfuel cell (DMFC) liquid fuel has an electrical resistance of 5×10⁵ Ω·cmor more and 1×10⁷ Ω·cm or less, preferably 1×10⁶ Ω·cm or more and 1×10⁷Ω·cm or less at 25° C.

An electrical resistance of a liquid fuel substantially composed of anaqueous methanol solution only can be easily measured by using acommercially available general electrical conductivity meter such asSC-82 (manufactured by Yokogawa Electric Corporation) or DS-50(manufactured by HORIBA, Ltd.). A DMFC liquid fuel having an electricalresistance of less than 5×10⁵ Ω·cm due to impurities therein provides anadverse effect of reducing an electromotive force in long-term operationof the fuel cell. In contrast, a DMFC liquid fuel having an electricalresistance of more than 1×10⁷ Ω·cm lacks components serving as a supplysource of hydrogen ions and thus is not an appropriate DMFC liquid fuel.

According to another embodiment of the present invention, a DMFC liquidfuel has a total content of components except methanol and water of 10ppm or less. Of the components in the liquid fuel, methanol and waterare essential components for taking out electrons and hydrogen ions atthe anode, and unconsumed methanol and water can be recovered. However,the components except methanol and water not only do not contribute toelectricity generation but also may provide adverse effects such asreduction in electromotive force when the components accumulate withinthe fuel cell. Thus, the liquid fuel preferably contains the componentsexcept methanol and water of 10 ppm or less. The components exceptmethanol and water of 10 ppm or less are considered to provide slighteffects of reduction in electromotive force.

The components in the liquid fuel except methanol and water, whichspecifically contribute to reduction in electromotive force, includeketone components, aldehyde components, halogen components, and alkalimetal components. Thus, according to still another embodiment of thepresent invention, a DMFC liquid fuel has a total content of ketonecomponents and aldehyde components of 5 ppm or less, preferably 4 ppm orless. The ketone components according to the present invention refer tocompounds of one kind or multiple kinds each having a ketone group in astructure. Similarly, the aldehyde components according to the presentinvention refer to compounds of one kind or multiple kinds each havingan aldehyde group in a structure. Examples of the ketone components andthe aldehyde components that may be actually included in the liquid fuelinclude acetone, acetaldehyde, and formaldehyde. The ketone componentsand the aldehyde components in the liquid fuel in a total content moreof than 5 ppm adhere to catalysts in electrodes, to thereby cause apoisoning phenomenon and thus reduction in electromotive force of thefuel cell.

According to yet another embodiment of the present invention, a DMFCliquid fuel has a total content of halogen components of 1 ppm or less,preferably 0.5 ppm or less. The halogen components in the liquid fuel ina total content of more than 1 ppm adhere to porous electrodes,catalysts, a proton conductive polymer electrolyte membrane, and othervarious members, to thereby cause inactivation, corrosion, or the likeand thus reduction in electromotive force of the fuel cell.

According to still yet another embodiment of the present invention, aDMFC liquid fuel has a total content of alkali metal components of 5 ppmor less, preferably 3 ppm or less. Examples of the alkali metalcomponents that may be actually included in the liquid fuel includelithium, potassium, and sodium. Magnesium components may also beincluded in the liquid fuel, and a total content thereof is preferablyreduced to about 5 ppm or less. The alkali metal components in theliquid fuel in a total content of more than 5 ppm adhere to the protonconductive polymer electrolyte membrane, to thereby increase internalresistance and thus reduction in electromotive force of the fuel cell.

Examples of a method of measuring contents of organic components such asketone components or aldehyde components in the liquid fuel include: aquantitative analysis method (GC/MS-SIM) by using a gaschromatography/mass spectrometry apparatus with selective ionmonitoring; and a combination of separation of the components throughcolumn chromatography represented by high performance liquidchromatography, and a compound specification method through IR, NMR, orthe like.

Examples of a method of measuring contents of inorganic components suchas alkali metal components or halogen components in the liquid fuelinclude: ion chromatography; emission spectrochemical analysis (ICPatomic emission spectrochemical analysis) utilizing inductively coupledplasma discharge; atomic absorption analysis; silver nitrate titration,ion selective electrode; and a combination thereof with ionchromatography.

Of various measurement methods, the contents of the components aremeasured through a quantitative determination method (GC/MS-SIM) and ionchromatography in the present invention. In the present invention, thecontents of the components in the DMFC liquid fuel except methanol andwater can be specified by combining the above-mentioned methods.

(Method of Producing Liquid Fuel According to the Present Invention)

A method known to a person skilled in the art may be used for the methodof producing the DMFC liquid fuel according to the present inventioncontaining as main components methanol and water. The present inventionhas features in electrical resistance of a DMFC liquid fuel and incontents of trace amounts of impurity components in the DMFC liquidfuel. Except those features, a composition, a production method, and thelike of the DMFC liquid fuel are not particularly limited. However, inconsideration of the fact that equimolar amounts of both methanol andwater are consumed at the anode, an amount of water in the liquid fuelis preferably 40 wt % or more and 99 wt % or less.

In order to obtain a such high purity liquid fuel, impurity componentsmust be removed from methanol and water as raw materials in advancethrough sufficient purification operations such as distillation,sublimation, and recrystallization, and then methanol and water must bemixed. Industrially used methanol generally contains about 30 ppm ofacetone and acetaldehyde in total. Industrial water generally containsabout 20 to 80 ppm each of components derived from chloride ions,potassium ions, and sodium ions. Purification methods of the rawmaterials are not particularly limited, and various examples thereofinclude distillation, use of an ion-exchange resin, and filterfiltration. In order to achieve the purity required in the presentinvention, the purification operations are preferably repeated severaltimes or combined.

(Direct Methanol Fuel Cell Cartridge According to the Present Invention)

The essence of the present invention is that the DMFC cartridgeaccording to the present invention stores one of the direct methanolfuel cell liquid fuels of the present invention. A shape of the DMFCcartridge, a fuel supply method into the cartridge, a method ofdetaching and attaching the cartridge from and to a fuel cell main body,and the like are not particularly limited.

However, in the DMFC cartridge of the present invention, a material fora member to be brought into contact with the liquid fuel preferablycontains no components soluble in methanol or water. Examples of thematerial for the member include a resin containing no plasticizer, ametal, and glass. In a case where the material for the member to bebrought into contact with the liquid fuel contains components soluble inmethanol or water, components eluted into the liquid fuel are mixed andaccumulated in the fuel cell, to thereby reduce electromotive force.

In the present invention, the phrase “a material for the cartridgecontains no components soluble in methanol or water” indicates thatsolubility of the material for the cartridge to methanol or water is 1ppm or less, preferably 0.1 ppm or less.

(Direct Methanol Fuel Cell System According to the Present Invention)

The essence of the present invention is that the DMFC system accordingto the present invention uses one of the liquid fuels of the presentinvention. Kinds of members of the DMFC system, a structure of thesystem, and a production method for the system are not particularlylimited.

However, in the DMFC system of the present invention, a material for amember to be brought into contact with the liquid fuel such as amaterial for an inner wall of a fuel cartridge or for a fuel passagepreferably contains no components soluble in methanol or water. In acase where the material for the member to be brought into contact withthe liquid fuel contains components soluble in methanol or water,components eluted into the liquid fuel are mixed and accumulated in thefuel cell, to thereby reduce electromotive force.

The fuel cartridge of the DMFC system of the present invention mayemploy the DMFC cartridge of the present invention. Alternatively, thefuel cartridge of the DMFC system of the present invention may have afunction of injecting a liquid fuel from the DMFC cartridge of thepresent invention.

Hereinafter, the present invention will be described in more detail byway of examples, but the present invention is not limited to theexamples without departing from the gist of the present invention.

EXAMPLE 1 (Production Example of Direct Methanol Fuel Cell Liquid FuelAccording to the Present Invention)

Commercially available first grade methanol was subjected topurification operation by using an ion-exchange resin once, filterfiltration operation by using a reverse osmosis membrane (allowingcompounds each having a molecular weight of 50 or less to passtherethrough) once, and purification operation through distillationunder reduced pressure once.

Then, tap water was subjected to purification operation by using anion-exchange resin three times, and purification operation throughdistillation under normal pressure twice. 5 parts by weight of purifiedmethanol and 95 parts by weight of purified water were mixed, to therebyobtain a direct methanol fuel cell (DMFC) liquid fuel of the presentinvention.

The liquid fuel of the present invention had an electrical resistance of2.5×10⁶ Ω·cm measured by using a conductivity meter (SC-82, manufacturedby Yokogawa Electric Company). The liquid fuel had about 0.1 ppm ofhalogen components measured by using an ion chromatograph (IC500,manufactured by Yokogawa Electric Company). The liquid fuel had about 1ppm of alkali metal components measured by similarly using an ionchromatograph. The liquid fuel had about 3 ppm of ketone components andaldehyde components in total measured by using GC/MS-SIM (5973N,manufactured by Agilent).

EXAMPLE 2 (Production Example of Direct Methanol Fuel Cell Liquid FuelAccording to the Present Invention)

Commercially available special grade methanol was subjected to filterfiltration operation by using a reverse osmosis membrane (allowingcompounds each having a molecular weight of 50 or less to passtherethrough) once, and purification operation through distillationunder reduced pressure once.

Then, commercially available purified water for chemical experiments wassubjected to purification operation by using an ion-exchange resin once,and purification operation through distillation under normal pressureonce. 15 parts by weight of purified methanol and 85 parts by weight ofpurified water were mixed, to thereby obtain a direct methanol fuel cell(DMFC) liquid fuel of the present invention.

The liquid fuel of the present invention had an electrical resistance of2.7×10⁶ Ω·cm, and 0.1 ppm or less of halogen components. The liquid fuelhad about 2 ppm of alkali metal components and about 4 ppm of ketonecomponents and aldehyde components in total.

EXAMPLE 3 (Production Example of Direct Methanol Fuel Cell CartridgeAccording to the Present Invention)

A vessel having an opening as a fuel supply port to an anode was formedof a propylene resin, and a polyester sponge member impregnated with theDMFC liquid fuel described in Example 1 was placed therein, to therebyobtain a DMFC cartridge of the present invention. The cartridge was leftstanding for 1 month, and then the liquid fuel inside the cartridge wasrecovered. The recovered liquid fuel was analyzed in the same manner asin Example 1, resulting in similar contents of the components exceptmethanol and water.

EXAMPLE 4 (Production Example of Direct Methanol Fuel Cell SystemAccording to the Present Invention)

A DMFC system having a structure shown in FIGURE was produced. Amembrane electrode assembly (MEA) 1 serving as an electricity generationpart of the DMFC system of Example 4 was provided with: an anode 101; acathode 103; and a proton conductive polymer electrolyte membrane 102arranged between the anode 101 and the cathode 103. In Example 4,“Nafion 112” (perfluorosulfonic acid membrane, available from DuPont)was used as the proton conductive polymer electrolyte membrane 102.Further, an anode passage plate 2 for supplying the liquid fuel to MEA 1was provided on a side of the anode 101. A fuel cartridge 4 storing theDMFC liquid fuel of the present invention was connected to the anodepassage plate 2 through a liquid supply pump 5.

In Example 4, the DMFC liquid fuel produced in Example 1 was stored inthe fuel cartridge 4. Meanwhile, a cathode passage plate 3 for supplyingan oxidizer gas to MEA 1 was provided on a side of the cathode 103. Agas supply pump 6 was connected to the cathode passage plate 3. MEA 1was heated by a heater (not shown).

A production example of MEA 1 is described below in more detail. 4 g ofconductive carbon (IEPC40A-II, available from ISHIFUKU Metal IndustryCo., Ltd.) carrying a catalyst (40 wt % Pt −20 wt % Ru) was mixed with10 g of ion-exchanged water and 8 g of a 5% Nafion solution (availablefrom Wako Pure Chemical Industries, Ltd.), to thereby obtain a paste.The paste was applied on a surface of “Nafion 112”, and the whole wasdried. An amount of a Pt—Ru alloy applied was about 4 mg/cm².

Next, carbon paper (TGP-H-060, available from Toray Industries, Inc.)having a thickness of 0.2 mm was attached to the applied surface of“Nafion 112”, and the whole was subjected to pressing, to thereby obtainMEA 1 of Example 4. An area of MEA 1 involved in electricity generationwas 25 cm².

In the DMFC system, the liquid fuel was supplied from the fuel cartridge4 to the anode passage plate 2 through the liquid supply pump 5, andpassed through grooves of the passage plate. In Example 4, a liquidsupply tube from the fuel cartridge 4 to the anode passage plate 2 wasformed of a polypropylene resin. A stainless steel member was used as aconnecting part between the fuel cartridge 4 and the liquid supply tube.

COMPARATIVE EXAMPLE 1 (Production Example of Direct Methanol Fuel CellSystem for Comparison)

5 parts by weight of commercially available first grade methanol(available from Kishida Chemical Co., Ltd.) and 95 parts by weight oftap water (Tokyo) were mixed, to thereby obtain a liquid fuel forcomparison.

The liquid fuel had an electrical resistance of 6.6×10³ Ω·cm throughconductivity measurement. The liquid fuel had about 1.5 ppm of halogencomponents, and about 20 ppm of alkali metal components through ionchromatography measurement. The liquid fuel had about 30 ppm of ketonecomponents and aldehyde components in total through a GC/MS-SIM method.

A DMFC system for comparison was produced in the same manner as inExample 4 except that the liquid fuel for comparison was stored in thefuel cartridge 4 shown in FIGURE. The DMFC system of the presentinvention produced in Example 4 and the DMFC system produced inComparative Example 1 were each subjected to electricity generationtesting for 720 hours in total. Table 1 shows open circuit voltages(electromotive force when current is not taken out) of each of the DMFCsystems before and after the testing.

TABLE 1 Open circuit voltage of DMFC systems Initial open Open circuitvoltage circuit voltage after 720 hours Example 3 0.90 V 0.89 VComparative 0.88 V 0.52 V Example 1

Table 1 shows that no significant deterioration of open circuit voltagewas observed in the DMFC system of the present invention, butelectromotive force of the DMFC system for comparison reduced with time.The difference is presumably caused by the components in each of theliquid fuels.

The present invention is described above by way of examples, but thepresent invention is not particularly limited to purification methodsfor raw materials or materials forming the cartridge or the system.

The present invention can provide a direct methanol fuel cell liquidfuel, a direct methanol fuel cell cartridge, and a direct methanol fuelcell system all appropriately used for stable electricity generationover a long period of time.

This application claims priority from Japanese Patent Application No.2004-312711 filed Oct. 27, 2004, which is hereby incorporated byreference herein.

1. A direct methanol fuel cell liquid fuel comprising methanol and waterto be supplied to an anode of a direct methanol fuel cell, wherein anelectrical resistance of the liquid fuel is from 5×10⁵ Ω·cm to 1×10⁷Ω·cm at 25° C.
 2. The direct methanol fuel cell liquid fuel according toclaim 1, wherein a total content of components in the liquid fuel exceptmethanol and water is from 4.1 ppm to 10 ppm.
 3. The direct methanolfuel cell liquid fuel according to claim 1, wherein a total content ofketone components and aldehyde components in the liquid fuel is from 3ppm to 5 ppm.
 4. The direct methanol fuel cell liquid fuel according toclaim 1, wherein a total content of halogen components in the liquidfuel is from 0.1 ppm to 1 ppm.
 5. The direct methanol fuel cell liquidfuel according to claim 1, wherein a total content of alkali metalcomponents in the liquid fuel is from 1 ppm to 5 ppm.
 6. A directmethanol fuel cell cartridge, which stores a direct methanol fuel cellliquid fuel containing methanol and water to be supplied to an anode ofa direct methanol fuel cell, wherein the liquid fuel has an electricalresistance of 5×10⁵ Ω·cm to 1×10⁷ Ω·cm at 25° C., wherein a material fora member in the fuel cell cartridge to be brought into contact with theliquid fuel is made of only a resin containing no plasticizer, a metal,or glass.